Convert docstring plot directives to doctests for plot.py

[mcflugen]

This pull request converts the plot directives in the plot.py module to doctests.

ref: #170

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.05%. Comparing base (9c86fbe) to head (face15b).
Report is 4 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #175      +/-   ##
===========================================
+ Coverage    88.83%   89.05%   +0.22%     
===========================================
  Files            9        9              
  Lines         3617     3617              
===========================================
+ Hits          3213     3221       +8     
+ Misses         404      396       -8     

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